Imagine yourself with pen and paper, writing a
letter to a friend far away. You finish the letter and sign it, reflect
on what you've written, then tuck the letter into an envelope.
You put your friend's address on the front, your return address
in the left-hand corner, and a stamp in the right-hand corner,
and the letter is ready for mailing.
Electronic mail - ail for short) is prepared in much the same way, but
a computer is used instead of pen and paper.

The post office transports real letters in real envelopes, whereas
sendmail
transports electronic letters in electronic envelopes.
If your friend (the recipient) is in the same neighborhood
(on the same machine), only a single post office (
sendmail
running locally)
is involved. If your friend is distant, the mail message will
be forwarded from the local post office (
sendmail
running locally) to a
distant one (
sendmail
running remotely) for delivery.
Although
sendmail
is similar to a post office in many
ways, it is superior in others:

Delivery typically takes seconds rather than days.

Address changes (forwarding) take effect immediately, and
mail can be forwarded anywhere in the world.

Host addresses are looked up dynamically. Therefore, machines can
be moved or renamed, and email delivery will still succeed.

Mail can be delivered through programs that access
other networks (such as UUCP and BITNET). This would be
like the post office using United Parcel Service to deliver an
overnight letter.

This analogy between a post office and
sendmail
will break
down as we explore
sendmail
in more detail.
But the analogy serves
a role in this introductory material, so we will continue to use it
to illuminate a few of
sendmail
's more obscure points.

A mail user agent (MUA) is any of
the many programs that users run to read, reply to, compose,
and dispose of email.
Examples of an MUA include the original UNIX mail
program (
/bin/mail
); the Berkeley
Mail
program; its System
V equivalent (
mailx
); free software programs such as
mush
,
elm
, and
mh
; and commercial programs such as
Zmail
.
Many MUAs may exist on a single machine.
MUAs sometimes perform limited mail transport, but
this is usually a very complex task for which they are not suited.
We won't be covering MUAs in this book.

A mail transfer agent (MTA)
is a highly specialized program that delivers mail and transports it
between machines, like the post office.
Usually, there is only one MTA on a machine.
The
sendmail
program is an MTA.
Others include
MMDF
,
Smail 3.x
, and
Zmailer
,
but we'll cover only
sendmail
in this book.